I think the right one can help a little bit, but I also think as far as especially the verbal portion goes, a student with self-discipline can get the official SAT blue book and study on his/her own. I think taking a few practice tests can prepare the student for the types of questions there will be. My daughter took an inexpensive prep test through a local community college, and basically all they did was use the blue book. She said it was more helpful for her for the math section, because they covered some concepts she was a little rusty on. Really, I think learning test taking strategies helps more than trying to learn new material though.
One thing you might consider, especially if like we are, you are in an area where not many students take the ACT, is to try the ACT also. Some students tend to do better on it...I know my dd did. If your child does take the ACT, we found a strategy between the first and second time she took it that greatly improved her science score. Do NOT read the science passages first-read the questions first, then skim the passage for the answer. If you really need to, you can read the passage afterwards, but I asked my daughter to try this method the day before she took the test, and she said it was remarkably effective. Her science score, which had been pretty darn decent the first time, took a pretty significant jump, and she is NOT a science person, by any means. HTH.
eta: I wouldn't spend a lot for a prep couse, though. Some of them are ridiculously expensive.